The invention relates to a method for the interferometric measurement of non-rotationally symmetric wavefront errors on a specimen, which can be brought into a plurality of rotational positions.
More specifically the in ration refers to a method for the interferometric absolute measurement of non-rotationally symmetric wavefront errors on a specimen which can be brought into a plurality of rotational positions.
From the general prior art and general practice, such methods for the interferometric absolute measurement of non-rotationally symmetric wavefront errors of optical surfaces in reflection and optical elements in transmission are known. One established method is the so-called rotational position test with n equidistant rotations through 360°/n for absolute measurement of the non-rotationally symmetric errors of a specimen. Such a method is described, for example, by “R. Freimann, B. Dörband, F. Höller: “Absolute Measurement Of Non-Comatic Aspheric Surface Errors”, Optics Communication, 161, 106-114, 1999”.
Evans and Kestner show in “C. J. Evans, R. N. Kestner: “Test Optics Error Removal”, Applied Optics, Vol. 35, 7, 1996” that generally, with n rotational positions, if these are averaged out correspondingly over the measurements, all non-rotationally symmetric errors with the exception of the orders k·n can be established absolutely, where k=1,2,3 . . . In general, this remaining residual error of the order k·n becomes commensurately smaller as more rotational positions are measured. There is great interest in developing fast and effective methods which permit a more efficient improvement [lacuna] analysis of the wavefront error.
JP 8-233552 has accordingly described a method in which, using mathematical methods, further points are determined in addition to the measured points in order to improve the accuracy if possible.
A more extensive method based thereon is described by U.S. Pat. No. 5,982,490. According to the “Third modified Example” described therein, four measurement values, which are arranged at predetermined non-equidistant spacings from one another are taken on a specimen. Through mathematical operations, further values are determined from these four measurement values, so that it is finally possible to achieve an evaluation accuracy which would otherwise have required the measurement of eight individual values at an equidistant spacing.
The disadvantage of this method is, however, that there are only four concrete measurement values here, which cover only half the circumference of the specimen, the other values being inherently “virtual” measurement results.